President Donald Trump warned ousted FBI Director James Comey on Friday against talking to the media, suggesting there might be tapes of conversations between the two men that could contradict his account.

"James Comey better hope that there are no 'tapes' of our conversations before he starts leaking to the press!" Trump said in a string of Twitter posts.

In a report that added to critics' charges that Trump has overstepped the norms of his office, the New York Times reported that the president asked Comey in January to pledge loyalty to him, a request that would undermine the standing of the FBI chief as an independent law enforcer.

On Twitter, Trump appeared to suggest that if Comey gave his version of contacts between them, the administration might produce tapes of conversations, although it was not clear if such tapes exist. The veiled threat added to the storm over Trump's abrupt firing of Comey on Tuesday.

Critics have assailed Trump for dismissing the FBI chief at a time when the agency is investigating alleged Russian meddling in the 2016 U.S. election, and possible collusion between Moscow and the Trump presidential campaign.

The Federal Bureau of Investigation probe and parallel congressional investigations have cast a cloud over Trump's presidency since he took office on Jan. 20, threatening to overwhelm his policy priorities.

Democrats have accused Trump of trying to dent the FBI probe by firing Comey, and have called for a special counsel to investigate the Russia issue.